GSTM1 and neoplasm: They include higher tumor stage [8, 9], pre-operative chemotherapy [4], radiotherapy [10–12], presence of comorbidity [13–16], American Society of Anesthesiologists (ASA) score [17, 18], concurrent neck dissection [9, 11, 19], low hemoglobin (Hb) level [19], low serum albumin concentration [14, 20, 21], perioperative blood transfusion [22], type of the flap [3, 17, 23] and operation time [17].